1. Field of the Invention
The present invention relates to a method for producing a copolymer for lithography to be used for production of a semiconductor, and a method for purifying a polymerization initiator for production of the copolymer. More particularly, the present invention relates to a method for producing a copolymer for semiconductor lithography with a reduced amount of metal impurities, which is suitable for obtaining a semiconductor having excellent electrical properties, and a purification method for obtaining a polymerization initiator with a reduced amount of metal impurities, for production of the copolymer.
2. Description of Related Art
In lithography used for production of a semiconductor, formation of a finer pattern is increasingly demanded due to increase in the integration density, and, at present, lithography techniques with KrF excimer laser light (with a wave length of 248 nm) or ArF excimer laser light (with a wavelength of 193 nm) are used for mass production. Further, research and development are in progress also for lithography techniques using F2 excimer laser light, which has a shorter wavelength (a wavelength of 157 nm), EUV (extreme ultraviolet) and X-ray, which have still shorter wavelengths than those excimer lasers, and the electron beam.
As miniaturization of the patterns proceeds, reduction of the amount of impurities contained in copolymers to be used for semiconductor lithography has been increasingly demanded. Since, in particular, metal impurities adversely affect production of semiconductors in various ways, they need to be removed as much as possible. For example, in cases where metal impurities such as sodium and iron are contained in a copolymer for a chemical amplification resist, the metal components capture an acidic substance generated from an acid generator during exposure, and a copolymer as a base component is therefore not sufficiently dissolved, so that a desired pattern cannot be formed. Further, in cases where metal impurities contained in a copolymer for semiconductor lithography, which is not restricted to a resist copolymer and may be a topcoat copolymer, antireflection coating copolymer or the like, finally remain on the surface of a semiconductor substrate, electrical properties of the semiconductor are impaired, leading to decrease in the yield of the product.
Reported examples of the method for removing metal impurities from copolymers for semiconductor lithography include a method wherein a copolymer is subjected to extraction with an organic solvent and water to distribute the copolymer into the organic layer and the metals into the aqueous layer, followed by removal of the aqueous layer (Patent Document 1) and a method wherein, to a solution of an alicyclic hydrocarbon polymer in an organic solvent, a poor solvent for the polymer and an acid are added to cause coagulation of the polymer, and the coagulated polymer, water-insoluble organic solvent, acid and water are mixed together to extract metals (Patent Document 2). Further reported examples of the method include a method wherein a novolak resin solution is allowed to pass through a cation-exchange resin and anion-exchange resin washed with deionized water and a mineral acid solution (Patent Document 3), a method wherein a dispersion prepared by dispersing a polymer in a dispersion medium is filtered through a filter such as a filter cloth preliminarily washed with an acidic aqueous solution, to obtain wet polymer powder with a reduced metal content (Patent Document 4) and a method wherein a polymer solution is allowed to pass through an absorbent such as a clay intercalation compound, active carbon or silica gel, to remove metals (Patent Document 5). Other reported examples include a method wherein, to a resist polymer solution, a water-soluble compound capable of forming a complex is added at an equivalent larger than that of metal purities in the polymer to complete the reaction, followed by washing the reaction product with pure water (Patent Document 6). However, operations in these methods are laborious, and it has been difficult to apply the methods to commercial-scale production of copolymers.
On the other hand, as an alternative to the methods in which metals are removed after production of a copolymer, there is a method wherein a raw material with a reduced metal content is used to reduce metals contained in a copolymer. Reported examples of such a method include a method wherein monomers as a raw material of a photoresist polymer is obtained by thin film distillation, which monomers contain each of Na, Mg, K, Ca, Mn, Fe and Cu with a content of not more than 50 ppb (Patent Document 7), a method wherein a (meth)acrylic acid ester having a cyclic skeleton such as an adamantane skeleton or lactone skeleton is subjected to molecular distillation or adsorption treatment with a chelate resin, to obtain a (meth)acrylic acid ester having a cyclic skeleton, whose metal content is not more than 500 ppb (Patent Document 8) and a method wherein monomers are washed with ultrapure water (Patent Document 9).
By the way, a polymerization initiator is also an important raw material. The molecular weight of a copolymer for semiconductor lithography is often relatively small as a macromolecule. Therefore, the amount of a polymerization initiator to be used is relatively large, and is often as large as several mol % to ten and several mol % with respect to the total amount of the raw material monomers. In particular, an azo polymerization initiator sometimes contains metals at a high concentration, which metals are derived by the production process of the polymerization initiator. Therefore, there is a concern that the metals largely affect the metal content of the copolymer produced using the polymerization initiator. As a method for reducing the metal content of a polymerization initiator, there is a method (Patent Document 10) wherein water is added to a solution of an azo polymerization initiator in a water-insoluble solvent, to extract metals into the aqueous layer, followed by removing the metals. In this case, it has been reported that the sodium content is reduced from 3300 ppb to not more than 500 ppb with respect to the polymerization initiator by one time of the extraction operation. However, there are problems in that, for example, this method requires additional steps and produces additional waste fluid.